Road maps are proposed in the art in which the magnitude and the direction of the gradient of a road are indicated on the map for a given interval measured along the length of the road to allow the presence of slopes and the degree of gradients to be readily recognized (see Japanese Laid-Open Patent Application No. 131,561/2003, for example).
In such a road map, a flat road (level terrain) is shown in blue, a slope with a medium gradient in yellow, and a slope with a sharp gradient in red, for example. This affords an advantage that an easy path can be chosen by following roads which are indicated in blue while avoiding roads indicated in red which represent a sharp gradient or upward slope.
However, it will be noted that in the road map mentioned above, the color is chosen in accordance with the magnitude of the gradient, for example, a sharp gradient is indicated in red independently from the direction in which a user desires to move, and therefore, the user has to determine whether the red indication represents an upward or a downward slope with respect to his intended course.
More specifically, when there are a number of downward slopes with sharp gradients when viewed in the direction of the course of a user, the individual downward slopes will be indicated in red. In this instance, the user can learn intuitively that there are a number of sharp slopes, but the user had to determine whether the sharp slope indicated in red represents a downward slope or an upward slope with respect to his course and to select an indicated road when it has a number of downward slopes. However, it is more troublesome to render such a determination when there are a number of slopes of different gradients (accordingly, indicated in different colors) in admixture.